@wmchiew,
thank you for being helpful. Now, I understood the problem more than before. It was a coincidence. I was drawing a triangle and by using glScalef(), I've noticed that the triangle disappeared because I was using glIdleFunc(). What I noticed that the scale function scales something that already scaled, consequently, the triangle disappeared. To overcome this problem, I used glPushMatrix() or ModelView and glLoadIdentitiy(). I got it now.